<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <div class="search-box">
    <a class="search-btn">
      <i class="fa fa-search" aria-hidden="true"></i>
    </a>
    <input type="text" class="search-txt" placeholder="搜索" />
    <div class="search-line"></div>
  </div>
</body>
<style>
  body {
    margin: 0;
    padding: 0;
    width: 100%;
    height: 100%;
    background: #f8cb59;
}
.search-line {
    position: absolute;
    left: 62px;
    bottom: 10px;
    width: 0px;
    height: 2px;
    background-color: rgb(251, 121, 0);
    transition: 0.3s;
}
.search-box {
    position: absolute;
    bottom: 30px;
    left: 30px;
    background-color: white;
    box-shadow: 0 2px 25px 0 rgba(0, 0, 0, 0.1);
    height: 40px;
    padding: 10px;
    border-radius: 40px;
    display: flex;
}
.search-txt {
    border: none;
    background: none;
    outline: none;
    padding: 0;
    color: #222;
    font-size: 16px;
    line-height: 40px;
    width: 0;
    transition: 0.4s;
}
.search-btn {
    color: #888888;
    font-size: 24px;
    width: 40px;
    height: 40px;
    border-radius: 50%;
    display: flex;
    justify-content: center;
    align-items: center;
    cursor: pointer;
    transition: 0.4s;
}
.search-box:hover .search-txt {
    width: 200px;
    padding: 0 12px;
}
.search-box:hover .search-btn {
    background-color: #fff;
    animation: rotate 0.4s linear;
}
.search-txt:focus {
    width: 200px;
    padding: 0 12px;
}

.search-txt:focus + .search-line {
    width: 200px;
}

@keyframes rotate {
    0% {
        transform: rotate(0deg);
    }
    100% {
        transform: rotate(360deg);
    }
}

</style>
</html>